Outdoorsman during periods of low ambient temperatures need a heat source to maintain body temperature. This is particularly true when confined to a small area, such as a blind, a tree stand or a water craft. A person trying to photograph a moose for example, in it's natural habitat is unlikely to see the animals if the animals can see, hear or smell the outdoorsman's presence. Heat sources that burn fuel and are currently available, produce odors, and noise that a moose can detect from a distance. The scents and noise often keep animals away from view by the outdoorsman.
Fuel burners generally produce high concentrations of carbon monoxide. This deadly gas overcomes a few outdoorsmen each year.